Recent years have witnessed many improvements in coding and decoding for digital communications systems. U.S. Pat. No. 4,133,976, issued on Jan. 9, 1979; RE 32,580, issued on Jan. 19, 1988; 4,701,954, issued on Oct. 27, 1987; 4,472,832, issued on Sep. 18, 1984, and 4,827,517, issued on May 2, 1989, to B. S. Atal, et al and assigned to the assignee of the present invention, all present important improvements in this field.
One area of such improvements have came to be called code excited linear predictive (CELP) coders and are, e.g., described B. S. Atal and M. R. Schroeder, "Stochastic Coding of Speech Signals at Very Low Bit Rates," Proc. IEEE Int. Conf. Comm., May 1984, p. 48.1; M. R. Schroeder and B. S. Atal, "Code-Excited Linear Predictive (CELP): High Quality Speech at Very Low Bit Rates," Proc. IEEE Int. Conf. ASSP., 1985, pp. 937-940; P. Kroon and E. F. Deprettere, "A Class of Analysis-by-Synthesis Predictive Coders for High-Quality Speech Coding at Rate Between 4.8 and 16 Kb/s," IEEE J. on Sel. Area in Comm SAC-6(2), Feb. 1988, pp. 353-363, and the above-cited U.S. Pat. No. 4,827,517. Such techniques have found application, e.g., in voice grade telephone channels, including mobile telephone channels.
The prospect of high-quality multi-channel/multi-user speech communication via the emerging ISDN has increased interest in advanced coding algorithms for wideband speech. In contrast to the standard telephony band of 200 to 3400 Hz, wideband speech is assigned the band 50 to 7000 Hz and is sampled at a rate of 16000 Hz for subsequent digital processing. The added low frequencies increase the voice naturalness and enhance the sense of closeness whereas the added high frequencies make the speech sound crisper and more intelligible. The overall quality of wideband speech as defined above is sufficient for sustained commentary-grade voice communication as required, for example, in multi-user audio-video teleconferencing. Wideband speech is, however, harder to code since the data is highly unstructured at high frequencies and the spectral dynamic range is very high. In some network applications, there is also a requirement for a short coding delay which limits the size of the processing frame and reduces the efficiency of the coding algorithm. This adds another dimension to the difficulty of this coding problem.